Generally, when bioprocess or pharmaceutical companies want to blend materials to produce a specific material they utilize a mixing container, such as a steel tank mixer or a disposable mixing system. These disposable systems, or single use devices are preferred over stainless steel tanks with their inherent high labor and materials cost for cleaning, because considerable savings in operating and capital cost. The use of single-use devices also minimizes the risk of product carryover and cross-contamination.
However, there are several problems associated with current disposable mixing systems. First, a major problem with developing a disposable mixing system is the manufacturing of a reliable aseptic seal that is inexpensive enough to be discarded after just one use, such as a mixing device that utilizes a low cost plastic bushing seals with a rotating propeller shaft. These seals cannot be relied upon to provide aseptic operation essential for pharmaceutical operations. The cost of such bags with their complex internals is too high for mixer applications, and most of their utility is in high performance applications such as fermentation and cell culture where their high cost can be justified.
Next, magnetically coupled seals offer an alternative to the bushing seals, such as a mixing device with a stir bar placed in a plastic bag that is driven by an external magnetic drive. The advantage of this method is that the fluid inside the mixing bag is completely isolated from the drive. The disadvantage is that, due to economic concerns, the stir bar is very small compared to the container diameter, and consequently performs poorly as a mixer. Since the stirrer is situated at the bottom of the bag, most of the fluid circulation induced by the stir bar fails to get to the upper regions of the mixing bag. The amount of power that can be transmitted through the magnetic coupling is limited. Some efforts have been made to use superconducting magnets to improve the power transmission efficiency, but these are costly to operate, and require liquid nitrogen to maintain the superconducting operation. Scaling up disposable magnetically coupled mixers is quite difficult and the utilization of commercial systems over 100 liters is unlikely. In addition, the stir bar is typically discarded after a single use that leads to a high cost of disposables and a problem with environmental disposal of the rare-earth magnets used in such applications.
A number of attempts have been made to develop a sealless disposable mixer, such as a mixing bag with an oscillating disk mounted at the bottom. The disk is forced to oscillate in the vertical dimension and its movement induces a circulation flow. This device has failed to find any significant commercial application because the fluid motion rapidly diminishes towards the upper regions of the container. The mixing performance is even poorer if the liquid phase has a high viscosity. The problem is that the mechanism constrains the vertical motion of the disk. Thus despite the relatively large diameter of the disk, the amount of fluid moved every oscillation is too small for it to function as an effective mixer.
Next, there is a mixer with multiple mixing platforms in a bag with a vertical shaft with horizontal mixer platforms. The shaft is moved up and down to mix the contents of the bag. The shaft is fixed to the upper surface of the bag that eliminates the need for a rotary seal, but the maximum possible stroke length is small due to the maximum allowable deflection of the top surface. This leads to poor mixing performance. In addition, bulk of the liquid flow bypasses the mixer platforms along the side walls, also reducing mixer efficiency.
Therefore, there is a need for an apparatus that provides the user with a mixing system that has a good mixing performance and is efficient. Also, there is a need for a mixing system that preserves its hermetic integrity and does not require any type of seal.